Organic electroluminescent elements using electro-luminescence (hereinafter referred to as EL) of an organic material are formed by stacking organic layers, i.e., an organic carrier transport layer and an organic light emitting layer, between an anode and a cathode, and attract attention as a light emitting element capable of high-luminance emission by low-voltage direct-current drive.
The development of active-matrix display devices having thin-film transistors (hereinafter referred to as TFTs) for driving organic EL elements for pixels among display devices using the above-described organic EL elements has been pursued particularly energetically by considering their high-image-quality and long-lifetime characteristics.
FIG. 5 is a schematic perspective view of an active-matrix organic EL display device. Display pixels 502 are disposed in a matrix form on a substrate 501, and a pixel circuit for driving an EL element for each pixel is contained. A drive circuit 503 for driving each pixel is also contained on the periphery of the array of the pixels. These circuits are constituted by TFTs and wiring and are connected to external circuits through external lead terminals 504 to be driven. Ordinarily, upper surfaces of the pixel portion 502 and the drive circuit 503 are covered with a sealing member 505 such as a glass or metal plate bonded to the substrate 501 by an adhesive 506 to maintain a mechanical strength.
In the display device thus constructed, a flattening insulating film is provided in a state of covering the TFTs and wiring provided on the substrate, and organic EL-elements are provided on this flattening insulating film. The organic EL elements and the wiring are connected to each other via contact holes formed in the flattening insulating film.
As an example of the flattening insulating film, one formed by applying a resin material such as polyimide by spin coating is used as disclosed in Japanese Patent Application Laid-Open No. H10-189252 (patent document 1).
With this display device, however, there is a problem described below.
That is, the flattening insulating film formed by application such as spin coating has a high water absorbing property because it is made mainly of an organic material or contains an organic material. For example, a water absorption coefficient of a polyimide film formed by application of a commercially available coating liquid is up to 1 to 3%. Since the light emitting portion of the organic EL element used as a display element is made of an organic material, a disadvantage such as a reduction in luminance intensity or an increase in drive voltage can occur easily therein by absorption of water. In a case where a material having a high water absorbing property is used as the flattening insulating film as described above, therefore, water gradually released from the flattening insulating film seriously affects the display performance of the display element, resulting in failure to enable the display device to have long-term reliability. Because degradation of the display element by absorption of water proceeds even during the process of manufacturing the display device, the release of water from the flattening insulating film is also a cause of a reduction in yield of the display device.
To solve the above-described problem, an arrangement such as disclosed in Japanese Patent Application Laid-Open No. 2001-356711 (patent document 2), for example, has been proposed in which a coat layer 6b made of an inorganic material is formed on the surface of a flattening insulating layer 6a as a barrier against water released from an organic material constituting the flattening insulating layer 6a and other impurity gas components (see FIG. 6).
In the display device shown in FIG. 6, TFTs 2 of the bottom gate type (which may alternatively be of the top gate type) are provided in a matrix form on a substrate 1 made of a glass material, for example, and an insulating film 3 is formed in a state of covering the TFTs. On this insulating film 3, wiring 4 connected to the TFTs 2 via connection holes not illustrated is provided on the insulating film 3.
On the insulating film 3 is also provided an interlayer insulating film 6, the wiring 4 being embedded in the interlayer insulating film 6. The interlayer insulating film 6 is provided in a multilayer structure formed of a flattening insulating layer 6a and a coat layer 6b provided on the flattening insulating layer 6a. The flattening insulating layer 6a is formed of a material obtained by using an organic material such as SOG (Spin-on-Glass) or a resin material (e.g., a polyimide resin, an acrylic resin, or an organic silica film). The flattening insulating layer 6a is an applied film formed by an application method such as spin coating. The coat layer 6b is formed by using an insulating material having a gas barrier property such as to be capable of limiting the release of a gas from the flattening insulating layer 6a . An inorganic material such as silicon oxide, silicon nitride (Si3N4), amorphous silicon (α-Si) or aluminium oxide (Al2O3) is used to form the coat layer 6b in a single-layer or multilayer structure. The coat layer 6b has such a film thickness as to be capable of sufficiently effectively suppressing the release of a gas from the flattening insulating layer 6a. 
In the interlayer insulating film 6 formed in such a multilayer structure, connection holes 7 reaching the wiring 4 are provided. Side peripheral walls of the connection holes 7 are covered with the coat layer 6b , and the upper surface of the flattening insulating film 6a and the surface facing the interior of each connection hole 7 are completely covered with the coat layer 6b. 
Organic EL elements 10 are provided on the interlayer insulating film 6 in a state of being connected to the wiring 4. The organic EL elements 10 are, for example, of an upper-surface emission type such that an emitted light radiates from the side opposite to the substrate 1 side and are each constituted by a lower electrode 11 connected to the wiring 4 via the connection hole 7, an insulating layer 12 provided in a state of covering a peripheral edge of the lower electrode 11, an organic layer 13 provided on the lower electrode 11, and an upper electrode 14 and a transparent electrode 15 provided on the organic layer 13. The organic EL elements 10 may alternatively be of a transmission type such that light is taken out from the substrate 1 side.
The device is thus arranged to suppress the release of water from the flattening insulating film to the organic EL elements and to prevent degradation of the organic EL elements due to absorption of water during drive over a long time period.
(Patent document 1)
    Japanese Patent Application Laid-Open No. H10-189252(Patent document 2)    U.S. patent application Publication No. US2002/036462A1
Even an arrangement such as shown in FIG. 6 also has a problem described below.
As described above, the arrangement shown in FIG. 6 is effective in preventing penetration (or permeation) of water into the upper organic EL elements from the flattening insulating film (indicated by arrow 21 in FIG. 6). In designing the actual device construction, however, other water penetration paths that should be considered exist, and which are also shown in FIG. 6. That is, penetration of water from the flattening insulating film at a display region end surface (indicated by arrow 22 in FIG. 6), penetration of water from contacting atmosphere (indicating arrow 23 in FIG. 6), penetration of water from contacting atmosphere from the upper surface of the display region (indicated by arrow 24 in FIG. 6) and the like are taken into consideration. Further, the interpixel insulating layer 12 formed between adjacent pixels for the purpose of preventing short-circuit between the lower (pixel) electrodes and the upper electrodes is made mainly of an organic material or contains an organic material, as does the flattening insulating film. Therefore there is a need to prevent penetration of water from the interpixel insulating film as well as through the flattening insulating film (indicated by arrow 25 in FIG. 6). However, as is seen from the above description of the prior art, no measures have been taken against penetration of water through these paths in the above-described arrangement.